Diverse electronic systems are intended for accurately measuring the amplitude of signals from sensors and other sources. Many signal processing circuits, such as amplifiers and signal transmission circuits, introduce undesirable offsets into signals of interest. To deal with this problem, certain electronic components, e.g., operational amplifiers, have offset nulling means built in to compensate for these inherent errors.
In many instances the input signal itself may have some offset or background level, in which cases the desired information is the amplitude of signals relative to that off-set or background level. This is often the case when monitoring an environment when signals are generated by sensors in response to environmental changes, such as light intensities, temperature, flow, etc. In such cases the desired information is the variation from normal or ambient conditions, rather than the absolute amplitude value of the variable.
In other situations an offset is intentionally added to the signal of interest to facilitate processing of the signal. This, for instance, is the case when analog signals are being processed by electronic circuits powered from a single supply which may not allow the signal level to vary beyond the voltage value of the supply. To process a signal with this type of circuitry, an offset, often equal to half the magnitude of the supply voltage, is added to the signal in the initial circuit stages. This offset is then often required to be removed after the processing has been completed.
In order to make accurate measurements of the signal, a means for removing the above described and other offsets, is desired. There are many known means for removing such offsets and restoring the reference level or the baseline of the signal. Some of these have been disclosed in the referenced prior art.
If the offset is of a known and constant magnitude, it can be compensated for by adding to the signal a constant (DC) voltage of a polarity opposite to that of the offset, with the result that the amplitude of the signal of interest will be referenced to zero or some other desired level. This correction can be accomplished with a summing amplifier combining the necessary DC off-set with the input signal.
In cases when the offsets to be removed are constant in a time frame relative to the measurements to be made, but vary between the measurements, these offsets can be eliminated by manual adjustment of the level of the compensating signal to be injected.
A well-known technique is frequently used to deal with offsets automatically. To effectively apply this technique, the offsets should be static or varying on a time scale significantly longer than that of the desired signal. In this approach, the input signal is directed to a low-pass filter, the output of which is a constant or a slow varying (relative to the measurement time scale) DC signal. The output from the low-pass filter is subsequently subtracted from the original signal to remove the offset as described in U.S. Pat. No. 3,846,710 to Chapman, which is herein incorporated by reference. This method provides continuous compensation (i.e., removal) of the offset.
The baseline restoration of the signal using the preceding technique is adequate for applications where the required relative accuracy of signal amplitude measurements is low, e.g., digital signals or signals with very low duty cycles. For signals of higher duty cycles, significantly varying amplitudes, and/or signals that require extreme accuracy in measuring their amplitude, this approach is limited by errors in signal restoration.
The output of the low-pass filter nominally represents the average value of the signal. With very low duty cycle signals, this average very closely approximates the offset from zero, or reference, level. As the duty cycle of the signal increases, the time interval when the amplitude of the signal is not zero, also increases. Consequently, the accuracy with which the average signal coming from the low-pass filter approximates the ideal compensation level is reduced in relation to the increase in the signal duty cycle. Likewise, significant variation in the amplitude of the input signal results in a similar variation in the average value of the compensating signal, also producing errors for this type of signals.
In those cases when the duty cycle and/or signal amplitude variations are fairly constant over the time scale of the measurements, the residual error from compensation schemes using low-pass filters may be reduced or eliminated by the addition of a manual compensation to the signal following the automatic baseline restoration. Such manually injected compensating signal would be equal in amplitude but of opposite polarity of the residual offset. One such approach is described in U.S. Pat. No. 5,675,517 to Stokdijk, which is herein incorporated by reference. The compensating circuit disclosed in that patent, however, still leaves residual errors for signals which have varying duty cycles and/or amplitudes.
In some systems where the signal pulses of interest are related in a synchronous manner to a known clock source, the reference level may be sampled between the pulses of interest, i.e., when the signal is known to be at the reference level. This level is then extracted and stored for use in measuring the amplitude of the following desired signal pulse(s) by a switching system which connects the input signal to a level storage device, such as a capacitor, during the reference period between said pulses. This stored value is then used as the reference level for the measurement by subtracting it from the input signal prior to the measurement, or by other means. This type of approach is described for example by U.S. Pat. No. 3,684,378 to Lord, and by U.S. Pat. No. 4,553,848 to Rosicke et al. which are herein incorporated by reference in various applications. As mentioned, this compensation scheme is limited to situations where the signals of interest arrive at the input at a known time in a manner synchronous with a known clock signal. This approach, however, is of little value where signals arrive at the input of a measurement system randomly, as is the case, for instance, with flow cytometry apparatus and other particle detection systems.